In a fuel cell, electrical energy and heat are generated as a result of hydrogen (H2) and oxygen (O2) being combined in an electrochemical reaction. For this purpose, hydrogen and oxygen are fed to the fuel cell either in their pure forms or as hydrogen-containing first operating gas and as oxygen-containing second operating gas. While the fuel cell is operating, the hydrogen-containing first operating gas is passed into the anode gas space of the fuel cell, the hydrogen penetrating through the porous anode of the fuel cell and thereby reaching the electrolyte of the fuel cell.
In the same way, the oxygen-containing second operating gas is passed into the cathode gas space of the fuel cell, the oxygen penetrates through the porous cathode of the fuel cell and likewise reaches the electrolyte of the fuel cell. Depending on the design of the electrolyte, either hydrogen ions or oxygen ions penetrate through the electrolyte, so that on one side of the electrolyte oxygen and hydrogen combine in an electrochemical reaction to form water (H2O), with electrical energy and heat being released.
The anode, the electrolyte and the cathode of the fuel cell form a membrane, which like a wall separates the anode gas space from the cathode gas space of the fuel cell. If this membrane has a leak, for example in the form of a hole, while the fuel cell is operating, by way of example, oxygen flows uncontrollably into the hydrogen-containing anode gas space. This leads to an uncontrolled reaction between hydrogen and oxygen in the anode gas space, which may form so much heat that the fuel cell is destroyed. Therefore, during production of the fuel cell it must at all costs be ensured that the membrane-like wall between the anode gas space and the cathode gas space does not have any leaks.
To discover leaks in the membrane of fuel cells, the fuel cells are subjected to a leak test before being delivered. In this context, it is efficient for a multiplicity of fuel cells to be combined to form a fuel cell module and to be tested together. The fuel cell module includes the fuel cell block which includes the cells as well as supply units and in particular a safety device which ensure that, in the event of an uncontrolled reaction between hydrogen and oxygen in a fuel cell, the fuel cell block is switched off in order to prevent the module from being destroyed.
If a leak is detected in a cell of a module, the fuel cell block which includes the fuel cell has to be removed from the module and dismantled, so that the damaged cell can be removed or exchanged. This is a highly complex method, since the module which has already been fully assembled has to be taken apart again. The significantly less complex variant of carrying out the leak test method on the “naked” fuel cell block without safety means is very risky, since in the event of a leak it is impossible to detect and suppress an uncontrolled reaction in a fuel cell. This can lead to substantial destruction within the fuel cell module.